Pipes of polymer material are frequently used for various purposes, such as fluid transport, i.e. transport of liquid or gas, e.g. water or natural gas, during which the fluid can be pressurised. Moreover, the transported fluid may have varying temperatures, usually within the temperature range from about 0° C. to about 50° C.
Polymer pipes are generally manufactured by extrusion, or, to a smaller extent, by injection moulding. A conventional plant for extrusion of polymer pipes comprises an extruder, a die-head, a calibrating device, cooling equipment, a pulling device, and a device for cutting or for coiling-up the pipe.
The manufacture of polyethylene materials to be used in pressure pipes is discussed for example in an article by Scheirs et al [Scheirs, Bohm, Boot and Leevers: PE100 Resins for Pipe Applications, TRIP Vol. 4, No 12 (1996) pp. 408-415].
WO 0022040 discloses a pipe having good mechanical properties made from a bimodal resin.
However, in very tough soil conditions, particularly when rock impigments can be feared, as well as under very hard installation conditions, it is desirable to have a pipe material having excellent SCG properties (slow crack growth properties) that is much higher as the SCG properties than presently found in materials in the state of the art. The specific conditions require a pipe material which has a SCG notch test of more than 5,000 hours, preferably 8,000 hours.
At the same time a good balance of processability, impact strength, modulus of elasticity, the rapid crack propagation resistance, and design stress rating of the pipe is required. The present invention solves the problem of providing pipes made from a multimodal material having a unique balance of the above mentioned properties.